1. Field of the Invention
The present invention relates generally to micro-fluid ejection devices and, more particularly, to a planar heater stack and methods for making the planar heater stack.
2. Description of the Related Art
Micro-fluid ejection devices have had many uses for a number of years. A common use is in a thermal inkjet printhead in the form of a heater chip. In addition to the heater chip, the inkjet printhead basically includes a source of supply of ink, a nozzle plate attached to or integrated with the heater chip, and an input/output connector, such as a tape automated bond (TAB) circuit, for electrically connecting the heater chip to a printer during use. The heater chip is made up of a plurality of resistive heater elements, each being part of a heater stack. The term “heater stack” generally refers to the structure associated with a portion of the thickness of the heater chip that includes first, or heater forming, strata made up of resistive and conductive materials in the form of layers or films on a substrate of silicon or the like and second, or protective, strata made up of passivation and cavitation materials in the form of layers or films on the first strata, all fabricated by well-known processes of deposition, patterning and etching upon the substrate of silicon. The heater stack also has one or more fluid vias or slots that are cut or etched through the thickness of the silicon substrate and the first and second strata, using these well-known processes, and serve to fluidly connect the supply of ink to the heater stacks. A heater stack having this general construction is disclosed as prior art in U.S. Pat. No. 7,195,343, which patent is assigned to the same assignee as the present invention. The entire disclosure of this patent is hereby incorporated herein by reference.
Despite their seeming simplicity, construction of heater stacks requires consideration of many interrelated factors for proper functioning. The current trend for inkjet printing technology (and micro-fluid ejection devices generally) is toward lower jetting energy, greater ejection frequency, and in the case of printing, higher print speeds. A minimum quantity of thermal energy must be present on an external surface of the heater stack, above a resistive heater element therein, in order to vaporize the ink inside an ink chamber between the heater stack external surface and a nozzle in the nozzle plate so that the ink will vaporize and escape or jet through the nozzle in a well-known manner.
During inkjet heater chip operation, some of the heating energy is wasted due to heating up the “heater overcoat”, or the second strata, and also heating up the substrate. Since heating or jetting energy required is proportional to the volume of material of the heater stack that is heated during an ejection sequence, reducing the heater overcoat thickness, as proposed in U.S. Pat. No. 7,195,343 is one approach to reducing the jetting energy required. However, as the overcoat thickness is reduced, corrosion of the ejectors or heater elements becomes more of a factor with regard to ejection performance and quality. So this patent proposes the additional steps of applying a sacrificial layer of an oxidizable metal on the resistive heater layer and then oxidizing the sacrificial layer to convert it to exhibit a protective function rather than a conductive function and thereby obviate the potential corrosive impacts of reducing overcoat thickness.
However, with the overcoat thickness decreasing, heat loss to the substrate then becomes the dominant factor. Thus, there is a need for an innovation that will reduce the heat loss to the substrate.